An electrohydraulic actuator for large aircraft is known in accordance with U.S. Pat. No. 4,426,911, entitled "Rotary Digital Electrohydraulic Actuator" of Curtis W. Robinson and Eugene T. Raymond, assigned to The Boeing Company, and issued Jan. 24, 1984. The actuator according to the '911 patent includes an axial piston hydraulic motor and a speed reduction gear set receiving torque from the motor and communicating this torque to an output shaft. A stepper motor driven commutating valve is provided to control fluid pressure and flow to and from the axial piston hydraulic motor.
An actuator according to the '911 patent offers a high level of torque output in comparison with the size and weight of the actuator, and provides a very high stiffness with respect to unwanted back driving movements of the output shaft by the load member. Unfortunately an actuator according to this patent is not suited for application upon relatively small airborne vehicles because of its weight, complexity, and cost. Also, relatively small airborne vehicles frequently do not have a hydraulic system which would be required in order to power an actuator as taught by the '911 patent.
A pneumatic actuator which is small and lightweight is known in accord with the teaching of U.S. Pat. No. 4,649,803, entitled "Servo System Method and Apparatus, Servo Valve Apparatus Therefore, and Method of Making Same", of Steven G. Abel, assigned to the same assignee as the present application, and issued Mar. 17, 1987. The actuator taught by the '803 patent employs a pair of unequal opposed pistons working on opposite sides of a toggle arm driving an output shaft. The smaller of the two pistons receives pressurized elastic fluid from a source thereof, while the larger of the two pistons receives a modulated pressure via a three-way closed-center solenoid valve.
An actuator according to the '803 patent is small, lightweight, and relatively simple in its construction. Also, this actuator may use pressurized gas from a simple storage vessel, and is well suited for use aboard small airborne vehicles. However, an actuator according to this teaching has a relatively limited output torque and limited stiffness with respect to undesirable back driving movements or oscillations of the load member. Also, this actuator may experience an undesirably high consumption of pressurized fluid when designed with a view to providing the stiffness needed by some airborne vehicles.
Yet another fluid valve actuator is known in accord with U.S. Pat. No. 4,533,890, entitled "Permanent Magnet Bistable Solenoid Actuator", of Balkrishna R. Pattell, assigned to General Motors Corporation of Detroit, Mich., and issued Aug. 6, 1985. The bistable solenoid actuator according to the '890 patent includes a pair of spaced apart opposed cup-like coil members receiving therebetween an annular armature which is movable between first and second bistable positions adjacent to either one of the two coil members. The armature includes a pair of oppositely disposed annular magnets and magnetically permeable material providing a flux path through the magnets. The pair of spaced apart coil members receiving the magnetic armature member therebetween are arranged so that they may simultaneously provide a repulsive force from one of the coil members and an attractive force toward the other of the coil members. Thus, the armature member is driven from the one bistable position towards the other position in response to an input command.
The bistable solenoid valve actuator taught by the '890 patent is in no respect proportional in its actions. This device is fast acting, and has essentially a step function output expressed as the position of a driven member connected with the armature.
Finally, a linear force motor is known in accord with the teaching of U.S. Pat. No. 4,631,430, of Richard A. Aubrecht, assigned to Mooge Incorporated of East Aurara, N.Y., and issued Dec. 23, 1986. The linear force motor in accord with the '430 patent includes a body defining an annular chamber receiving an armature, the body and the armature cooperating to define four axially spaced active air gaps across which magnetic flux originating with a pair of permanent magnets is conducted. The permanent magnet flux may be moderated by control flux from a pair of electromagnet coils arranged upon the body. The control flux from the electromagnets when superimposed upon the flux from the permanent magnets creates a net flux imbalance which urges the armature to move in a desired direction relative to the body. While this linear force motor of the '430 patent does provide a output of linear movement in accord with the current flow in the electromagnetic coils thereof, the armature itself is of such a nature that its connection to the valve member of a valve device is not easily accomplished.